usr/src/usr.bin/cksum/cksum.1

.\" Copyright (c) 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" %sccs.include.redist.man%
.\"
.\"     @(#)cksum.1     5.1 (Berkeley) %G%
.\"
.TH CKSUM 1 ""
.UC 7
.SH NAME
cksum \- display file checksums and block counts
.SH SYNOPSIS
.ft B
.nf
cksum [file ...]
.fi
.ft R
.SH DESCRIPTION
The
.I cksum
utility calculates and writes to standard output a total number of bytes,
a checksum CRC and the file name for each input file (or the standard input
by default).
.PP
The CRC used is based on the polynomial used for CRC error checking in
the networking standard ISO 8802-3: 1989.
The CRC checksum encoding is defined by the generating polynomial:
.sp
.RS
G(x) =
.RS
x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
.RE
.RE
.PP
Mathematically, the CRC value corresponding to a given file is defined by
the following procedure:
.RS
.PP
The
.I n
bits of the file are considered to be the coefficients of a mod 2
polynomial M(x) of degree
.IR n -1.
.PP
M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by
G(x) using mod 2 division, producing a remainder R(x) of degree <= 31.
During the division, each time the intermediate remainder is zero, it
is changed to the next value from a predefined sequence of 32-bit
integers before completing the division.
This sequence is long and complex -- see the source code for more
information.
.PP
The coefficients of R(x) are considered to be a 32-bit sequence.
.RE
.PP
The calculation used is identical to that given in pseudo-code in
the ACM article listed below.
.PP
The
.I cksum
utility exits 0 on success, and >0 if an error occurs.
.SH SEE ALSO
.IR "Computation of Cyclic Redundancy Checks Via Table Lookup" ,
Dilip V. Sarwate, Communications of the ACM, August 1988.
.SH STANDARDS
.I Cksum
is expected to conform to IEEE Std 1003.2 (``POSIX'').
.SH HISTORY
.I Cksum
first appeared in 4.4BSD.